An envelope of a cathode ray tube comprises a panel portion onto which an image is projected, a neck portion on which an electron gun is mounted and a funnel-shaped funnel portion which connects the panel portion and the neck portion to each other. The electron ray emitted by the electron gun hits a fluorescent material provided on the inner surface of the panel portion to allow the fluorescent material to emit light so that an image is projected onto the panel portion. During this process, damping X-rays are produced in the tube. When these damping X-rays leak out of the tube through the envelope, they have adverse effects on the human body. Thus, this kind of an envelope has been required to have a high X-ray absorbing power.
In order to enhance the X-ray absorption coefficient of glass, glass may include PbO. However, the use of PbO-containing glass as panel glass is disadvantageous in that electron rays emitted when an image is projected cause coloring called browning, making the image less viewable.
In order to inhibit browning, it has been practiced to use glass containing SrO or BaO, which exhibits a high X-ray absorption coefficient, rather than PbO, which causes browning, as panel glass.
Recently, higher applied voltage has been used to obtain an image having higher brightness and higher quality. It has thus been desired to provide panel glass with a higher X-ray absorption coefficient.
However, when SrO or BaO is contained in glass, liquid phase temperature of glass is likely to increase. Therefore, when the content of SrO or BaO is increased to thereby obtain a high X-ray absorption coefficient, glass can has easily deposition of devitrification stones due to strontium or barium during forming, and thus forming of the glass becomes difficult. As a result, it has been disadvantageous in that panel glass having a desired X-ray absorption coefficient cannot be produced with a good yield.
Furthermore, when the applied voltage is increased, browning is easily caused in even glass free of PbO.
In recent years, images having two kinds of frame ratios, i.e., 4:3 and 16:9, have been broadcasted. Thus, the panel portion has an area where an image is always projected and an area where an image is not projected depending on the frame ratio. Accordingly, periods of time for irradiation with electron rays are different in the two areas. As a result, amounts of browning are different in the two areas. In particular, it is difficult to obtain images on the border of the two areas.